Field of the Invention
The invention relates to a method for operating a nuclear magnetic flowmeter for determining the flow of a medium flowing through a measuring tube, in particular for determining the flow of a medium with slug flow, having a measuring device, wherein the measuring device contains an RF circuit having external electronics and having at least one RF coil designed for generating an excitation signal for exciting the medium and/or for detecting measuring signals emitted by the medium.
Description of Related Art
Nuclear magnetic flowmeters are designed for determining the flow of a medium flowing through a measuring tube. The medium can contain one phase or several phases. In the case of a single-phase medium, determination of the flow occurs by determining the flow velocity of the medium. Determining the flow of a multi-phase medium includes, in addition to determining the flow velocity, also determining the portions of the individual phases in the medium.
A prerequisite for using nuclear magnetic measuring methods is that the medium or each phase of the medium has atomic nuclei with magnetic moments. It is additionally necessary for distinction of individual phases that the phases have different relaxation times.
The multi-phase medium extracted from oil sources consists essentially of the two liquid phases crude oil and water and the gaseous phase natural gas, wherein all three phases contain hydrogen atoms that have a magnetic moment.
The medium flowing through the measuring tube can have different flow characteristics. This means that the individual phases of the medium can be unevenly distributed over a volume element. A flow that occurs often in media extracted from oil sources is slug flow. The slug flow has very complicated characteristics and is essentially characterized by a continuous disruption of a steady flow. A slug flow can be separated into two zones that occur alternately. The first zone is characterized by a slug of liquid medium, wherein the liquid medium fills the entire cross section of the measuring tube. Small gas bubbles are unevenly distributed in the liquid medium. This zone is called the slug zone. The second zone consists of a large gas bubble that occupies a dominating portion of the measuring tube cross section and a liquid film that fills the remaining part of the measuring tube cross section. This zone is called the film zone. The expansion of both alternating zones is arbitrary and irregular.
Nuclear magnetic flow measurements are wherein an excitation pulse exciting the medium is generated in external electronics, that the excitation pulse is fed into the medium using a RF coil and that the response of the excited medium is detected with a RF coil and also transmitted to external electronics for processing. A nuclear magnetic flowmeter thus includes at least one external electronic device and a RF coil with medium flowing through it, which together form a RF oscillation circuit. Hereby, the medium flowing through the RF coil must be considered as an additional load in the RF oscillation circuit.
The frequency of the RF circuit is set using so-called tuning so that resonance is achieved, i.e., the maximum possible signal.
In order to guarantee a maximum transmission of power between the external electronics and the RF coil, the impedance of the coil must be adjusted to that of the external electronics. This procedure is known as matching. An incorrect or inexact matching leads to the power not being completely transmitted, but rather partially reflected. If the power is reflected, not enough power is provided to the coil. This leads to the magnetic field B1 generated by the RF coil not having the intended strength and, thus, the magnetization of the medium being deflected at a value smaller than the intended value.
An inadequate matching additionally leads to difficulties in the detection of measuring signals emitted by the sample. The measuring signal emitted by the sample, which is usually very small, “appears” even weaker when incorrectly “matched”, which leads to a very bad signal to noise ratio.
In order to guarantee optimum measuring conditions, the tuning and matching have to be set before each nuclear magnetic flow measurement.
As already described, the settings for matching are dependent on the impedance of the RF coil. The additional load occurring due to the medium is taken into consideration in the value for the impedance of the RF coil. The load of the medium depends on the characteristics of the medium. If a medium having a slug flow characteristic flows through the measuring tube, the additional load in the RF oscillation circuit when a slug zone is located in the RF coil is different than when a film zone is located in the coil.
Consequently, the settings for matching have to be corrected for each zone, which is practically impossible or very complex and time consuming.
It is known from practice to use complex data analysis algorithms for data evaluation, which are able to differentiate between the two zones. These algorithms, however, are very complex, whereby data evaluation is time-consuming and additionally error-prone.